The present invention generally relates to an apparatus for use with a subscriber line interface circuit in a telephone system. More particularly, the present invention relates to a method and apparatus for detecting an AC electrical characteristic of the subscriber line in the presence of said time vowing ringing signal and providing a ring trip indication in response to the AC electrical characteristic reaching a predetermined threshold.
A telephone system generally includes one or more subscriber telephone devices, each subscriber telephone device being coupled with a telephone central office by a subscriber line. The subscriber telephone devices may be telephones or other telephone equipment. The telephone central office handles switching of telephone signals between subscriber telephone devices. Industry standards govern the electrical signal levels used for switching, coupling and signaling within the telephone system.
When a telephone call is placed to a particular subscriber telephone device, the telephone central office must send signals to the telephone device to indicate the incoming call. The telephone central office sends ringing signals which cause the subscriber telephone device to generate a ringing indication. The ringing indication may be a ringing bell, an electronic tone or some other audible or visible indication of ringing. The telephone central office applies the ringing signals directly to the subscriber telephone line.
Each subscriber telephone device is coupled to the telephone central office by a subscriber line. The subscriber line generally includes two conductors, one labeled xe2x80x9ctipxe2x80x9d and the other labeled xe2x80x9cring.xe2x80x9d Tip conductors and ring conductors carry both AC (time varying) and DC signals. The subscriber telephone device plus, the associated tip and ring conductors is commonly known as a loop or subscriber loop.
The subscriber telephone device can be modeled electrically as a resistor, an inductor and a capacitor in series and selectively coupled by a switch to the tip and ring conductors. The switch is known as the hook switch. When the telephone is on hook, and not in use, the hook switch couples the resistor-inductor-capacitor combination between the tip and ring conductors. When the telephone is off hook, or in use, the switch couples the tip and ring conductors through a resistor only. By detecting the D.C. impedance between the tip and ring conductors, the telephone central office can determine whether the subscriber telephone device is off hook (in use) or on hook (not in use).
When providing ringing signals to an intended receiving subscriber telephone device in response to a call originated by another (call originating) subscriber telephone device, the telephone central office must be able to determine whether the intended receiving subscriber telephone device is on hook or off hook for several reasons. First, if the intended receiving subscriber telephone device is off hook, the telephone central off ice must not send ringing signals, but must rather send a busy tone to the call originating subscriber telephone device. Secondly, if a user answers the phone by taking the receiving subscriber telephone device off hook in response to the ringing signal, the telephone central office must detect the change from on hook to off hook so that the receiving telephone does not ring loudly in the user""s ear, causing the user discomfort. Further, in response to the receiving telephone going off hook, the telephone central office completes the connection between the call originating telephone device and the receiving subscriber telephone device. The process of providing ringing signals and detecting the on hook or off hook state of the receiving subscriber telephone device by the telephone central office is known as ring trip detection.
There are numerous industry standards governing ring trip detection. The telephone central office, or other equipment providing the ringing signals to the subscriber lines, must detect that the receiving subscriber telephone device has gone off hook and terminate the ringing signals within a predetermined time period, such as 200 msec. Further, the telephone central office or other device which provides ringing signals to the subscriber line must be able to provide a ringing indication to any telephone coupled to the subscriber line. The telephone central office or other device providing the ringing signals should provide no false ring trip detections and should not miss any ring trip detections. Still further, the device which provides the ringing signals to the subscriber line must work with any length phone line, including both short (low impedance) or long (high impedance) subscriber lines.
Short subscriber lines couple the telephone central office to subscriber telephone devices which are physically near the central office, perhaps just a few blocks away. Long subscriber lines couple the telephone central office to subscriber telephone devices which are physically distant from the central office, perhaps miles away. Long subscriber lines have a greater impedance, measured from the central office, than short subscriber lines. Long subscriber lines are also more susceptible to noise due to coupling from adjacent noise sources such as other subscriber lines and power lines, than are short subscriber lines.
One industry standard requires the ringing signal to be applied as an AC voltage with a DC offset. The AC voltage, measured at the telephone central office, is preferably a 90 volt rms, 20 Hz, AC signal in order to provide at least 40 volts vms at the receiving telephone device. The DC offset is preferably 48 volts, measured at the central office. The ringing signal is generally applied as an unbalanced or single-ended ringing signal. That is, the AC signal is applied to either the tip conductor or the ring conductor with either polarity of DC offset (referred to as ring-plus, ring minus, tip-plus and tip-minus ringing). The conductor to which the AC and DC ringing signals are not applied is grounded in an unbalanced ringing design. The goal when applying ringing signals is to place the 90 volt rms AC signal across the tip and ring conductors.
When the subscriber telephone device is on hook, no DC path exists in the subscriber telephone device to permit DC current to flow in response to the applied DC offset voltage. However, with the telephone on hook, alternating current may flow in response to the applied AC ringing voltage to cause the subscriber telephone device to generate a ringing indication. When the subscriber telephone device is taken off hook, a DC path is established to couple the tip and ring conductors and allow DC current to flow in the loop. The central office detects the flow of the DC current in the loop to determine that the subscriber telephone device has been taken off hook and interrupts the ringing signal. Thus, prior art telephone apparatus have detected ring trip by detecting the DC impedance between the tip and ring conductors, typically by measuring DC current flow in response to a known applied DC voltage.
Application of the 90 volt rms signal and the 48 volt DC offset to the subscriber line means that the ringing generator must be able to handle potential differences substantially equal to 250 volts. It has heretofore been uneconomical to fabricate a ringing generator using a silicon integrated circuit.
Silicon integrated circuits which can handle 250 volts are expensive. Individual circuit elements, such as transistors, resistors and capacitors, designed to handle such large voltages must be physically large in order to sink and source the large currents associated with such large voltages. This large size requires substantial xe2x80x9creal estatexe2x80x9d on the surface of an integrated circuit which reduces the scale of integration of such devices as well as reducing manufacturing yield. Both the large size and the reduced yield increase manufacturing cost.
Moreover, circuit elements which can handle up to 250 volts must be fabricated using a manufacturing process designed to produce devices having junction breakdown potentials in excess of this voltage. Devices with smaller junction breakdown potentials will not function properly and may be permanently damaged when subjected to such large voltages. Manufacturing processes for silicon devices of the type commonly used for logic and control functions do not provide sufficient junction breakdown potentials to handle such large voltages, so logic and control circuitry cannot be readily integrated with telephone ringing signal transmitting circuitry. This further increases manufacturing cost of the overall system.
Where a central telephone office must generate ringing signals, the 250 volt requirement is not prohibitive. The telephone central office may be coupled with many thousands of subscriber lines. A single ringing generator is needed to provide ringing signals to these many subscriber lines. Thus, the high cost of the equipment needed to generate the 250 volt ringing signals is shared by the many thousands of subscriber lines.
However, modern telephone systems are moving away from systems in which a central office supplies many thousands of subscriber lines directly. Rather, in modern systems the telephone central office is coupled by an optical fiber to an optical network unit (OKU). The ONU couples the digital signals carried by the optical fiber with the analog electrical signals carried by the subscriber line to the subscriber telephone device. A relatively small number of subscriber lines are coupled to the ONU, perhaps one to ten. The ONU is located physically close to the subscriber telephone devices serviced by this small number of subscriber lines, preferably in the same city neighborhood as the subscriber telephone devices. This is known as a fiber in the loop (FITL) system. In a FITL system, each ONU must be capable of generating ringing signals. Ringing signals are not carried on the fiber optic line; the fiber optic line only transmits a ring command from the central office to the ONU. In response to the ring command, the ONU must generate the appropriate ringing signal on the appropriate analog subscriber line. Because only one or a very few subscriber lines are coupled to an ONU, the high cost of a ringing generator capable of supplying 250 volts cannot be shared among a large number of subscriber lines, as was the case in the prior art central office telephone system. However, since the ONU is physically close to the subscriber telephone devices which it services, the ONU must only provide analog signals to a short subscriber line, which may be only a few city blocks in length.
Accordingly, there is a need in the art for an apparatus and method for generating ringing signals requiring a total voltage less than 250 volts and which will cause a subscribe telephone device to produce a ringing indication. Further, there is a need in the art for an apparatus and method for generating ringing signals using only a silicon device coupled with the subscriber line. Still further, there is a need in the art for an apparatus and method for detecting ring trip using a silicon device.
The invention provides a method for providing ringing signals to a subscriber telephone device coupled to a subscriber line, the subscriber line including a first conductor and a second conductor. The method includes the steps of providing a first time varying signal to the first conductor and a second time varying signal to the second conductor; detecting an AC impedance between the first conductor and the second conductor in the presence of the first time varying signal and the second time varying signal; and providing a ring trip indication when the AC impedance is below a predetermined threshold.
The invention further provides an apparatus for use with a subscriber line interface circuit in a telephone system, the subscriber line interface circuit being interposed between a telephone central office and a subscriber line, the subscriber line being coupled with a subscriber telephone device. The apparatus comprises an interface means for coupling the apparatus with the subscriber line; a ringer means coupled with the interface means for providing a time varying signal to the subscriber line in response to a received control signal; and detect means coupled with the interface means for detecting an impedance of the subscriber line in the presence of the time varying signal and providing a ring trip indication when the impedance is below a predetermined impedance threshold.
The invention further provides an apparatus for coupling a subscriber line with a digital signal path in a telephone system, the apparatus receiving information including a ring conmnand from the digital signal path, the subscriber line being coupled with a subscriber telephone device. The subscriber device includes signal means and hook switch means. The subscriber line has a first conductor and a second conductor and the hook switch means has a first state for coupling the first conductor with the second conductor and a second state for decoupling the first conductor and the second conductor. The signal means generates a ringing indication in response to a ringing signal on the subscriber line when the hook switch means is in the second state. The apparatus comprises interface means for coupling the apparatus with the subscriber line; ringer means coupled with the interface means for providing the ringing signal to the subscriber line, the ringing signal preferably including only a first time varying signal and a second time varying signal, the ringer means providing the first time varying signal and the second time varying signal in response to the ring command. The apparatus further includes detect means coupled with the interface means for detecting an AC impedance between the first conductor and the second conductor in the presence of the ringing signal, the AC impedance having a first value when the hook switch has its first state and a second value when the hook switch has its second state, the detect means providing a ring trip indication when the AC impedance has one of the first value and the second value. The interface means, the ringer means, the detect means are preferably integrated in a common integrated circuit.
The invention still further provides an apparatus for use with a subscriber line interface circuit in a telephone system, the subscriber line interface circuit being interposed between a telephone central office and a subscriber line, the subscriber line being coupled with a subscriber telephone device and having a first conductor and a second conductor. The apparatus comprises a first interface circuit coupled with the subscriber line; a second interface circuit coupled with the telephone central office and receiving a ring command from the telephone central office; a ringing generator coupled with the first interface circuit, the ringing generator providing a first ringing signal to the first conductor and a second ringing signal to the second conductor in response to the ring command, the first ringing signal and the second ringing signal being balanced, time varying signals. The apparatus further comprises an impedance detector coupled with the first interface circuit, the impedance detector detecting an AC impedance between the first conductor and the second conductor in the presence of the first ringing signal and the second ringing signal, the impedance detector providing a ring trip indication when the impedance is below a predetermined impedance threshold. The first interface circuit, the second interface circuit, the ringing generator and the impedance detector are preferably integrated in a common integrated circuit, and the subscriber telephone device produces a ringing indication in response to the first ringing signal and the second ringing signal.
The invention still further provides an improved subscriber line interface circuit for coupling to a subscriber line. The subscriber line is coupled with a subscriber telephone device, the subscriber line having a first conductor and a second conductor. The subscriber line interface circuit includes a first interface circuit coupled With the subscriber line; a second interface circuit coupled with the telephone central office and receiving a ring command from the telephone central office; a ringing generator coupled with the first interface circuit, the ringing generator providing a first ringing signal to the first conductor and a second ringing signal to the second conductor in response to the ring command, the first ringing signal and the second ringing signal being balanced, time varying signals; and an impedance detector coupled with the first interface circuit, the impedance detector detecting an AC impedance between the first conductor and the second conductor in the presence of the first ringing signal and the second ringing signal, the impedance detector providing a ring trip indication when the impedance is below a predetermined threshold. The first interface circuit, the second interface circuit, the ringing generator and the impedance detector are integrated in a common integrated circuit. The subscriber telephone device produces a ringing indication in response to the first ringing signal and the second ringing signal.
It is therefore an advantage of the present invention to provide an apparatus and method for use in a subscriber line interface circuit which generates ringing signals for a subscriber telephone device and detects an off hook state of the subscriber telephone device.
A further advantage of the present invention is to provide ringing signals at a lower total voltage, relative to the prior art.
Yet a further advantage of the present invention is to provide a ringing generator which may be economically integrated in a silicon integrated circuit.
Yet a further advantage of the present invention is to provide ringing signals capable of ringing any telephone within the constraints of industry standards.
Further advantages and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawing illustrating the preferred embodiment of the present invention.